Structural beams which can withstand both bending and torsion loads are useful in a variety of applications, including industrial, aerospace, and orthopedic uses. One type of structural beam which has received considerable attention in the orthopedic field is a hip-joint replacement device. In basic design, this device includes an elongate curved stem which is adapted for receipt in a cavity formed in the proximal region of a femur, and a ball-like joint member carried on a neck at the upper end of the stem. When implanted in operative position, the device functions as a load transfer member between the pelvis and femur, and as such, must accommodate considerable bending, axial compression and torsional forces applied across the joint to the femur.
Three basic constructions have been proposed hereafter for hip-joint devices of this type. In all of these constructions, the curved stem, which is adapted for insertion into a bone cavity, and the neck, which is adapted to support the ball-like joint member, are formed as a single piece, and the joint member is separately attached to the neck, preferably after inserting the stem into the bone. In one construction the stem and neck are formed as a unitary metal piece from stainless steel or, more preferably, from a titanium alloy. An advantage of an all-metal construction is that the relatively thick metal stem and neck provide adequate bending and shear strength, so that problems of stem fracture or fatigue are minimal. A disadvantage of the construction is a high degree of weight loading stress on certain regions of the bone, and stress protection in other bone regions. Both high stress and stress protection can cause bone deterioration and resorption, leading to areas of bone weakness and loss of bone support for the prosthesis.
The related problems of bone stress and stress protection which can occur in a hip-joint replacement can be understood from the mechanics of weight load transfer across the hip-joint device. Normally, much of the weight load is transferred to the femur near the upper joint region and this load is distributed to and carried by the underlying cortical bone region and the prosthesis stem. The distribution of forces in the underlying cortical region and prosthesis stem region is determined by the relative stiffness--or elastic modulus--of the bone and stem respectively. In normal bone, the elastic modulus of the outer cortical bone region is about 2.5, and that of the softer interior cancellous region is less than 1, so that weight loading forces are carried primarily by the outer cortical region. By contrast, the metal stem region of a prosthetic device, which replaces the soft cancellous region of bone, has an elastic modulus typically between about 15-35, so that much more weight loading is carried by the stem, and much less by the outer cortical bone. In addition to the stress protection this produces in the bone region adjacent the stem, the high-modulus stem also produces unnaturally high bone stress at the lower tip of the stem, where forces carried in the stem are transmitted to the bone.
In a second known prosthesis construction, the stem and neck are formed from rolled or laminated layers of a composite material containing oriented carbon fibers embedded in a polymer resin. This construction is described generally in PCT patent application for "Orthopedic Device and Method of Making Same", WO No. 85/04325, filed Mar. 29, 1985. In a preferred embodiment, a series of composite layers containing fibers oriented in different directions are laminated, according to known composite block construction methods, to produce a machinable block whose different fiber orientations confer strength in different, selected directions with respect to the long axis of the block. The laminated block is then machined to produce a stem and neck piece which can be implanted in bone and fitted with a ball-like joint member. Since the laminate structure has a somewhat lower average elastic modulus, both in tension and shear, then a comparable-size metal prosthesis, the above problems related to stress protection along the length of the prosthesis stem, and the high concentration of forces at the lower tip of the stem are somewhat reduced. However, the effective elastic moduli of the stem in tension and shear is still very high compared with the soft cancellous region of bone which the stem has replaced. Furthermore, the laminate material is generally not as strong as a comparable-size metal stem, particularly at the neck region of the device where weight loading is borne entirely by the prosthesis. This is due in part to the fact that the carbon fibers oriented lengthwise in the stem do not follow the curvature of the stem, and generally do not extend along the entire length of the stem.
A third prothesis construction which has been proposed in the prior art involves a metal core having a relatively large-diameter neck and small-diameter stem which is encased in a low-modulus polymer. A prosthesis of this type is described by Mathis, R., Jr., et al in "Biomechanics: Current Interdisciplinary Research" (Perren, M., et al, eds.) Martinus Nijhoff, Boston (1985) pp. 371-376. The combined modulus of the polymer and inner core of the device is much more like that of interior cancellous bone than is either a solid metal or laminate composite structure, and as a result, problems related to bone stress protection and high stress are reduced. This compound device has not been entirely satisfactory, however. One problem which has been encountered is fracturing at the neck/stem interface, due to large loading force applied at this juncture by the neck. A second problem is related to the cutting action of the relatively stiff metal core against the low-modulus polymer, in response to forces exerted on the stem in directions normal to the stem's long axis. Over an extended period, the cutting action can lead to core wobbling within the bone, and exaggerated movement of the core in response to loading.
Another general problem which has been encountered in prior art hip-joint replacement devices is poor seating and fixation of the stem in the bone cavity. This problem has been discussed at length in the above-cited copending patent application. Briefly, the size and shape of prior art hip prosthesis devices requires removal of a substantial amount of hard outer cortical bone in forming the stem-receiving cavity, and this can weaken the bone structure and reduce blood supply to the proximal femur. In addition, the relatively large prosthesis cross section and lack of natural bone support for the prosthesis makes it difficult to anchor the prosthesis by press fit in the cavity. As a result, the stem may work loose in time, due to torsional stresses.